1. Field of the Invention
The present invention relates to an optical writing device and an optical writing method.
2. Description of the Related Art
In an image forming apparatus that uses an electrophotographic method to form an image, image formation is performed in such a manner that a semiconductor laser irradiates a static electric charge formed on a photosensitive drum with a laser beam, thereby forming the electrostatic latent image, and the electric latent image is then developed into an image by using a developer. A conventional semiconductor laser emits one to four laser beams or at most about eight laser beams from one semiconductor element. Recently, a surface emitting laser, referred to as a VCSEL, has become commercially available and been put to practical use. Accordingly, in recent years, an image forming apparatus has been proposed that uses a VCSEL and performs high-precision, and high-speed image formation and the like.
A VCSEL can emit about 40 laser beams from one chip. Therefore, it is thought that high-precision, high-speed image formation and the like are made possible by using a VCSEL to form latent images. In the case of using a VCSEL as a laser device for latent image formation, a latent image having adequate characteristics cannot be formed simply by replacing a semiconductor laser with a VCSEL. For example, a VCSEL generates a number of laser beams in a planar form from a predetermined light-emitting region. A laser device used for latent image formation needs to control the light quantity of the emitted laser beam so that a target light quantity is met. Furthermore, in the case of a VCSEL, to form a high-precision latent image stably, it is required to enhance the degree of integration of laser beams in the light-emitting region and manage the light quantity of the laser beams.
Meanwhile, a light source, such as a semiconductor laser or a VCSEL, is subjected to the phenomenon of degradation due to static electricity or long-term use. An indication of degradation may be that a light quantity becomes smaller than the initial quantity even after the same amount of current is applied to the light source or that the light quantity of emitted light does not increase in proportion to an increase in the current applied.
Furthermore, the existing technologies for controlling a laser beam to have a target light quantity are based on the assumption that a light source such as a semiconductor laser or a VCSEL is not degraded. Consequently, when a degraded light source is subjected to the control of the light quantity, although the controlling side intends to light with the light of the target light quantity, the actual quantity of light emitted from the light source may not be the target light quantity due to the degradation. Therefore, even if the number of laser beams is increased or the higher-precision control of the light quantity is made, when the light source is degraded, the precise control of the light quantity cannot be made. As a result, it is not possible to provide a high-precision image, and in the worst case, a defective image is output.
For these reasons, various technologies for detecting degradation of a light source have been proposed. For example, in a technology proposed in Japanese Patent Application Laid-open No. 2002-141605, a device for measuring a voltage value correlating with a drive current currently applied to a light source is provided. The current voltage value is compared with a preset voltage value, and it is determined that the light source is degraded if the current voltage value exceeds the preset voltage value.
In the technology proposed in Japanese Patent Application Laid-open No. 2002-141605, degradation of the light source is determined based on a voltage value common to all image forming apparatuses which are produced. However, in general, even when the quantity of emitted light of each semiconductor laser is the same, the drive current differs among the semiconductor lasers. Consequently, the degradation level of the light source differs among the image forming apparatuses. As a result, even when the degradation level of a semiconductor laser does not affect the print image quality, it is determined that the semiconductor laser is degraded and the print job may be aborted.
Furthermore, in a technology proposed in Japanese Patent Application Laid-open No. H10-083102, an amount of initial current when a light source lights with a predetermined quantity of light is stored in a recording medium, an amount of later current when the light source lights with the predetermined quantity of light is compared with the initial current amount stored in the recording medium, and it is determined that the light source is degraded if the later current amount is increased by a specified ratio with respect to the initial current amount. In this method, an initial drive current can be measured and held by each image forming apparatus individually, so degradation of a light source can be determined without any effect of variations of light sources.
However, in general, a light source, such as a semiconductor laser or a VCSEL, has temperature characteristics even in a state where the light source is not degraded. Consequently, for example, when the light source is controlled to be applied with the same amount of current, the light quantity may decrease if the temperature surrounding the light source rises. Therefore, a unit for controlling the light quantity of the light source increases the amount of current to raise the light quantity to a target light quantity. In the method disclosed in Japanese Patent Application Laid-open No. H10-083102, even an increase in the amount of current increased in this way is recognized as an increase caused by degradation of the light source. Consequently, even when the light source is not actually degraded, there is a possibility of determining that the light source is degraded.
Furthermore, in the methods disclosed in Japanese Patent Application Laid-open No. 2002-141605 and Japanese Patent Application Laid-open No. H10-083102, a device for detecting the drive current of a light source is required, and in the case of a light source that emits multiple of beams, such as a VCSEL, as many devices for detecting a drive current as the number of the beams are required, and therefore, there is a problem in that the circuit size is increased, for example, several times.